It is generally recognized that the production of noxious oxides of nitrogen (NO.sub.x) which pollute the atmosphere are undesirable and in many cases are controlled by limits established by local, state and federal governmental regulations. The formation of NO.sub.x constituents in the exhaust gas products of an internal combustion engine must, therefore, be eliminated, minimized or at least maintained below some predetermined limit or level.
It is generally understood that the presence of NO.sub.x in the exhaust of internal combustion engines is determined by combustion temperature and amount of NO.sub.x present in the engine exhaust. It is, therefore, desirable to pressure. An increase in combustion temperature causes an increase in the control the combustion temperature in order to limit the amount of NO.sub.x present in the exhaust of an internal combustion engine.
A common method utilized in the prior art for limiting or controlling the combustion temperature has been to recirculate a portion of the exhaust gas back to the engine air intake (so-called exhaust gas recirculation or EGR). Since the exhaust gas has a higher specific heat, the combustion mixture will burn at a lower temperature. The lower combustion temperature will, in turn, reduce the amounts of NO.sub.x produced during combustion.
Prior to widespread recognition of the problems caused by exhaust emissions, it was common practice to run an internal combustion engine at or near a spark timing which produces maximum peak combustion pressures. Unfortunately, however, unacceptably high levels of NO.sub.x are produced in the combustion chambers when the engine operates at or near spark timings which produce maximum peak combustion pressure. In order to inhibit the formation and emission of NO.sub.x, it is therefore desirable to limit the peak combustion pressure to a selected value.
EGR has been employed in the prior art for limiting combustion pressure since it is well known that an increase in recirculation of exhaust gases into the induction passage of the combustion chamber will reduce peak combustion pressure and thus the attendant levels of undesirable NO.sub.x.
Therefore, it is generally well known that the formation of undesirable oxides of nitrogen may be reduced by recirculating a portion of the exhaust gas back to the engine air/fuel intake passage so as to dilute the incoming air/fuel mixture with inert N.sub.2, H.sub.2 O, and CO.sub.2. The molar specific heat of these gases (especially CO.sub.2) absorbs substantial thermal energy so as to lower peak cycle temperatures and/or pressures to levels conducive to reducing NO.sub.x formation.
While NO formation is known to decrease as the EGR flow increases to where it represents about 20% of the exhaust gas constituents, it is also known that this is accompanied by a deterioration of engine performance including, but not limited to, an increase in engine roughness and a decrease of power output with increasing EGR. Therefore, one factor limiting the magnitude of EGR is the magnitude of EGR-induced performance deterioration or roughness that can be tolerated before vehicle driveability becomes unacceptable. Furthermore, EGR should not be turned on during load transients, as this causes "incomplete combustion" which results in black smoke from the engine exhaust. It is also usually desirable that EGR be turned off during hard acceleration so that the engine may operate at maximum power output.
Determining the proper amount of EGR under varying engine operating conditions has proved to be a complex and difficult task in the prior art. Most prior art control systems utilize at least two sensed engine parameters as inputs to the control system which controls the EGR. For example, U.S. Pat. No. 4,224,912 issued to Tanaka utilizes both engine speed and the amount of intake air as control variables. U.S. Pat. No. 4,142,493 issued to Schira et al. utilizes either engine speed and manifold absolute pressure or engine speed and throttle position. U.S. Pat. No. 4,174,027 issued to Nakazumi utilizes both clutch-actuation detection and throttle valve-opening detection as input variables to the control system. These methods all require the monitoring of several engine parameters, which may have a significant cost impact if the monitored signals are not readily available within the engine. It is, therefore, desirable to control the EGR with a single monitored engine parameter as input to the control system in order to reduce the complexity of the control system, thereby improving cost efficiency and system reliability.
Also, many of the prior art EGR control systems cannot be used with diesel engines. Diesel engines differ from spark ignition engines in a number of important ways, one being that the diesel engine does not include a valved, or throttled, intake manifold into which the combustion air is induced through a throttle and valve. Accordingly, the vacuum pressure existing in a diesel engine intake duct is slight at most. The source of vacuum pressure provided by the intake manifold of a spark ignition engine is, therefore, not available in a diesel engine. Hence, any prior art control system utilizing the vacuum pressure as an input to the control system will not work with a diesel engine.
In a diesel engine, the engine speed under a given load is controlled by the quantity of fuel injected into the engine combustion chambers and accordingly the "throttle" of the diesel engine is considered to be a manually operated foot pedal connected by a linkage to a fuel pump for supplying the engine fuel injectors. The foot operated pedal is actuated to govern the quantity of fuel delivered by the fuel pump to the combustion chambers of the engine and thus controls the engine speed under a given load. Since the quantity of fuel introduced into the combustion chamber varies, the production of NO.sub.x varies as a function of the throttle setting. This being the case, it is theoretically possible to control EGR in a diesel engine using only the throttle position as an input to the control system.
The present invention is therefore directed toward providing an EGR control system which utilizes only throttle position as an input to the control system. Such a control system could then be used with a diesel engine.